AAWSAP DIRD, Quantum Tomography of Negative Energy States in the Vacuum, January 2011

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Figure 14. Experimentally Measured Squeezed State. (courtesy of P.
Marecki) This graph of vacuum dB noise vs. relative optical phase angle shows
an experimentally measured squeezed state (plot (I)) and a normal
(undisturbed) vacuum state (plot (II)). The deep valleys with negative dB
values in plot (I) are sub-vacuum regions with sub-vacuum (negative) energy
density (see also, Figure 1 for a comparison).
When applied to pulsed sources, the frequency-domain BHD technique implies that
averaging over many individual laser pulses takes place. However, in time-domain BHD,
each laser pulse generates a signal that is observed in real time and yields a single
value of a field quadrature. Repeated measurements of a large number of laser pulses
produce a quantum probability distribution associated with this quadrature. When
transform-limited LO pulses are used, time-domain BHD gives the complete information
about the quantum state in the spatial-temporal mode that matches that of the LO.
Hansen et al. [4] point out that time-domain BHD is technically challenging, because 1)
the electronics must ensure time resolution of individual laser pulses and 2) the
measured quadrature values must not be influenced by low-frequency noise. The
detector must provide ultralow noise, high subtraction, and a flat amplification profile in
the entire frequency range from DC to at least the LO pulse repetition rate. See
Reference [4] for a complete description of their device as shown in Figure 13.
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